The benzoquinone ansamycin geldanamycin (GA) stimulates proteasome-mediated degradation of plasma membrane–associated ErbB2, a receptor tyrosine kinase. Drug sensitivity is mediated by ErbB2's kinase domain and occurs subsequent to the disruption of Hsp90 interaction with this domain. Full-length ErbB2 is efficiently processed via the endoplasmic reticulum (ER) and Golgi network, so that at steady state most of the detectable protein is plasma membrane associated. However, previous studies have also demonstrated the GA sensitivity of newly synthesized ErbB2, normally a minor component of the total cellular pool of the kinase. Drug sensitivity of nascent ErbB2 is distinguished by 2 characteristics—protein instability and inability to traverse the ER. As nascent ErbB2 can associate with both cytoplasmic Hsp90 and its ER luminal homolog Grp 94, also a GA-binding protein, the purpose of this study was to examine the relative contributions of the cytoplasmic and ER luminal domains of ErbB2 to the GA sensitivity of the nascent kinase. By studying the drug sensitivity of ErbB2/DK, a construct lacking ErbB2's cytoplasmic kinase domain, and by examining the activity of a GA derivative that preferentially binds Hsp90, we conclude that both the stability and the maturation of nascent ErbB2 are regulated by its cytoplasmic, Hsp90-interacting domain.